Radiation therapy may be used to treat a patient condition, such as cancer. For example, radiation therapy may be used to deliver ionizing radiation to a malignant tumor or other target tissue of the patient in order to control malignant or other abnormal cells. One or more external treatment beams, e.g., photon streams emitted by a radiation source, may be used to deliver the radiation to the target tissue of the patient. The area of the body through which an external radiation beam is directed to reach a tumor or other target tissue within the patient may be referred to as a “treatment field.” Each treatment beam may be controlled to minimize the radiation dose delivered to normal tissue surrounding the target tissue. If a critical structure (e.g., a radiation-sensitive organ or other radiation-sensitive tissue) is located proximate to the target tissue, each treatment beam may be configured to avoid the critical structure. For example, in some radiation therapy systems, the treatment beam or multiple treatment beam angles may be arranged and weighted to define a treatment field that encompasses the target tissue, while minimizing the number of critical structures within the treatment field.
Intensity modulated radiation therapy (IMRT) is one technique for reducing the radiation dose that is delivered to critical structures that lie within a treatment field of the radiation beam. In a modulated treatment field, some portions of the treatment field (i.e., subfields) receive higher doses than others. In some cases, a high dose gradient between subfields of unequal dose may be required to avoid a critical structure while still delivering a significant dose to the target tissue. A modulation device, such as a multileaf collimator (MLC), may be used to deliver specified doses to the treatment field and/or defined subfields of the treatment field.
A MLC includes a plurality of individually adjustable leaves positioned between a radiation source and a patient. Each of the leaves may be moved in and out of the path of the treatment beam to direct the treatment beam toward the patient and shape the beam to conform to the treatment field or a subfield of the treatment field previously defined by a treatment plan. The configuration of the MLC leaves may be changed throughout therapy delivery, e.g., during a radiation therapy delivery session, and a sequence of MLC configurations may be used to deliver varying radiation dosages to subfields defined by the MLC configurations.
Depending upon the location of the target tissue within the patient, the target tissue may undergo motion during delivery of the treatment beam. For example, tumors in the lungs, stomach, pancreas or liver may move in and out of the treatment field and/or defined subfields of the treatment field due to breathing-induced motion.